Method and apparatus for homogenizing a glass melt

ABSTRACT

The present invention is directed toward a method of reducing contamination of a glass melt by oxide particulates, such as particulates of platinum oxide, which may condense on the inside surfaces of a stir chamber, particularly the stir shaft, and fall back into the glass melt. The method includes causing a flow of gas through an annular space between the shaft and the chamber cover. In one embodiment, the flow of gas through the annular space is caused by drawing a vacuum in the chamber. An apparatus for practicing the method is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a method of reducing contaminants ina glass melt, and more specifically to reducing condensation-formedcontaminants during a glass stirring process.

2. Technical Background

Chemical and thermal homogeneity is a crucial part of good glass formingoperations. The function of a glass melting operation is generally toproduce glass with acceptable levels of gaseous or solid inclusions, butthis glass usually has cord (or striae or ream) of chemically dissimilarphases. These non-homogeneous components of the glass result from avariety of normal occurrences during the melting process includingrefractory dissolution, melting stratification, glass surfacevolatilization, and temperature differences. The resulting cords arevisible in the glass because of color and/or index differences.

One approach for improving the homogeneity of glass is to pass themolten glass through a vertically-oriented stir chamber locateddownstream of the melter. Such stir chambers are equipped with a stirrerhaving a central shaft which is rotated by a suitable motor. A pluralityof blades extend from the shaft and serve to mix the molten glass as itpasses from the top to the bottom of the stir chamber. The presentinvention is concerned with the operation of such stir chambers withoutintroducing further defects into the resulting glass, specifically,defects arising from condensed oxides.

Volatile oxides in a glass stir chamber can be formed from any of theelements present in the glass and stir chamber. Some of the mostvolatile and damaging oxides are formed from Pt, As, Sb, B, and Sn.Primary sources of condensable oxides in a glass melt include hotplatinum surfaces for PtO₂, and the glass free surface for B₂O₃, As₄O₆,Sb₄O₆, and SnO₂. By glass free surface what is meant is the surface ofthe glass which is exposed to the atmosphere within the stir chamber.Because the atmosphere above the glass free surface, and whichatmosphere may contain any or all of the foregoing, or other volatilematerials, is hotter than the atmosphere outside of the stir chamber,there is a natural tendency for the atmosphere above the free glasssurface to flow upward through any opening, such as through the annularspace between the stirrer shaft and the stir chamber cover. Since thestir chamber shaft becomes cooler as the distance between the stirrershaft and the glass free surface increases, the volatile oxidescontained with the stir chamber atmosphere will condense onto thesurface of the shaft if the shaft and/or cover temperature are below thedew point of the oxides. When the resulting condensates reach a criticalsize they can break off, falling into the glass and causing inclusion orblister defects in the glass product.

Heating the shaft above the glass free surface has proven only partiallysuccessful in reducing particulate contamination in the glass melt,resulting only in a stratification of the condensation.

One prior art method of reducing contamination of the glass melt bycondensates has been to dispose a disc-shaped shield between the glassfree surface and upper portions of the stir chamber. However, suchmethods may make it difficult to control the temperature of the glassfree surface, such as by heating the chamber cover above the glass. Inaddition, the joint between the shield and the stirrer shaft may serveas an additional source of condensate contamination.

SUMMARY

In one broad aspect of the invention, a method of stirring a glass meltis provided comprising flowing molten glass through a stir chamber, thestir chamber having at least one wall and a cover, the cover having apassage therethrough. The stir chamber further includes a stirrercomprising a shaft which extends through the cover passage, therebyforming an annular gap between the shaft and the cover. A gas is flowedthrough the annular gap at a rate of at least about 100 sccm (standardcubic centimeters per minute). Preferably the gas is flowed at a rate ofat least about 400 sccm, more preferably at least about 900 sccm andmost preferably at a rate of at least about 1200 sccm. The flow of gasis preferably everywhere downward through the annular gap. Preferably,the gas flows at a velocity of at least about 0.35 m/s for an annulargap of about 0.25 inches (0.635 cm). Preferably, the gas is air,although other gases may be used, as appropriate. The methodadvantageously causes the gas to flow along the stirrer shaft, therebyreducing the condensation of volatile oxides along the shaft at apredetermined rate, which may thereafter dislodge and contaminate themolten glass.

Also disclosed is a preferred apparatus for practicing the methoddisclosed herein (i.e. stirring a glass melt) comprising a stir chamberconfigured to hold molten glass and a stir chamber cover, the coverdefining a passage therethrough, a stirrer having a shaft extendingthrough the cover into the stir chamber, thereby forming an annular gapbetween the cover and the shaft, one or more gas flow tubes forevacuating the stir chamber, the flow tubes having an end within thestir chamber, the end of the flow tube not extending overtop a surfaceof the molten glass.

In an alternative embodiment of the apparatus, an annular spacer platemay be positioned between the cover and the stir chamber wall, thespacer plate comprising gas flow passages for evacuating or pressurizingthe stir chamber. The gas flow passages may be connected to a manifold.The manifold is in fluid communication with a vacuum system or with acompressor system, depending upon whether or not the stir chamber is tobe evacuated or pressurized with a gas.

The invention will be understood more easily and other objects,characteristics, details and advantages thereof will become more clearlyapparent in the course of the following explanatory description, whichis given, without in any way implying a limitation, with reference tothe attached Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplary stir chamber accordingto an embodiment of the present invention showing the chamber cover andthe gas flow pipes which enter the region of the chamber above the levelof the glass free surface.

FIG. 2 illustrates the offset between the center of the inside perimeterof the cover passage and the center of the stirrer shaft.

FIG. 3 is a cross sectional view of an exemplary stir chamber accordingto another embodiment of the present invention showing the stir chamber,the stir chamber cover, and a spacer plate disposed between the chamberand the chamber cover.

FIG. 4 is a horizontal cross section view of a spacer plate, including agas manifold, which may be used between the stir chamber wall and thecover, the spacer plate have passages for evacuating or pressurizing thestir chamber.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary apparatus for practicing a method forhomogenizing a glass melt according to an embodiment of the presentinvention. Stir chamber 10 of FIG. 1 includes an inlet pipe 12 and anoutlet pipe 14. In the illustrated embodiment, molten glass flows intothe stir chamber, as indicated by arrow 13, through inlet pipe 12, andflows out of the chamber, as shown by arrow 15, through outlet pipe 14.Stir chamber 10 includes at least one wall 16 which is preferablycylindrically-shaped and substantially vertically-oriented, althoughstir chamber may have other shapes such as oval or hexagonal.Preferably, the stir chamber wall includes an inner lining 18 comprisingplatinum or a platinum alloy. Other lining materials having similarrefractory properties, including resistance to corrosion, as well aselectrical conductivity, may be substituted. Glass inlet pipe 12 islocated at or near the bottom of stir chamber 10 whereas glass outletpipe 14 is located near the top of the stir chamber. However, it will berecognized by the skilled artisan that inlet pipe 12 and outlet pipe 14may be reversed, such that the molten glass flows into the stir chamberfrom the top and flows out through the bottom of the stir chamber.Intermediate positions for the inlet and outlet pipes may also beemployed provided adequate stirring (i.e. the desired amount ofhomogenization) is achieved.

Stir chamber 10 further includes a stirrer 20 comprising shaft 22 and aplurality of blades 24 which extend outward from the shaft towards wall16 of the stir chamber. Shaft 22 is typically substantiallyvertically-oriented and rotatably mounted such that blades 24 whichextend from the lower portion of the shaft rotate within the stirchamber at least partially submerged below free surface 26 of the moltenglass. The molten glass surface temperature is typically in the rangebetween about 1400° C. to 1600° C., but may higher or lower dependingupon the glass composition. Stirrer 20 is preferably composed ofplatinum, but may be a platinum alloy, or a dispersion-strengthenedplatinum or platinum alloy (e.g., a zirconia-strengthened platinumalloy).

As shown in FIG. 1, stir chamber 10 may include a drain tube 28 forremoving glass from the stir chamber during, for example, shut down ofthe system. In addition (or alternatively), the stir chamber may includean optional sump 30. Stirrer 20 is rotated by a suitable drive. Forexample, stirrer 20 may be rotated by an electric motor (not shown)through appropriate gearing or by a belt drive.

In accordance with the present embodiment, stir chamber 10 is covered bychamber cover 32. Chamber cover 32 may rest directly upon wall 16, orhigh temperature sealing material may be disposed between the wall andthe cover, the seal between the wall and the cover in any event beingsufficient to prevent appreciable gas flow between the cover and thewall. Cover 32 may also include cover heater 34 for heating the chambercover and therefore helping to control the free surface temperature ofthe glass melt flowing through the stir chamber. Cover heater 34typically includes a resistance coil, typically comprising platinum,imbedded within the chamber cover refractory material. The resistancecoil is supplied with an electric current, preferably alternatingcurrent, although direct current may be applied, to thereby heat thechamber cover. The chamber cover is typically between about 2 inches(5.08 cm) and 3 inches (7.62 cm) from the free surface of the glassmelt, but this distance may be greater, as needed. Thus, volume 35 isdefined between the stir chamber cover 32, stir chamber wall 16 andglass free surface 26.

Chamber cover 32 also includes a passage through which stirrer shaft 22passes. The inside surface of the passage may include a lining whichforms casing 36. As with other components of the stir chamber, it isdesirable that casing 36 be resistant to corrosion due to the hightemperature and the corrosive gases and condensates which may developfrom the molten glass. Casing 36 typically comprises platinum or aplatinum alloy. Shaft 22 passing through the chamber cover passage formsannular gap 38 between the outside surface of shaft 22 and the insidesurface of either the passage or, should casing 36 be employed, theannular gap is formed between the outside surface of the shaft and theinside surface of the casing. For the purpose of eliminating confusion,reference shall be made hereinafter only to the inside surface of thecasing, but shall be construed to mean both instances, whicheverapplies. That portion of shaft 22 above chamber cover 32 is surroundedby a refractory material containing shaft heater 40. Shaft heater 40, asin the case of cover heater 34, preferably comprises a resistanceheating element. The heating element is preferably comprised ofplatinum, but may be a platinum alloy.

An insulation layer 42 is disposed overtop chamber cover 32. Insulationlayer 44 similarly surrounds shaft heater 46. Annular gap 38 eliminatescontact between the rotating shaft and the casing, heaters, insulationand cover. Preferably, the center of the inside circular perimeter ofthe casing, i.e. the center circumscribed by the annular space, isoffset from the center of the shaft by no more than about 0.15 inches(0.381 cm), more preferably no more than about 0.12 inches (0.305 cm),and most preferably by no more than about 0.04 inches (0.102 cm). Thisis illustrated by FIG. 2 showing offset α between the shaft center andthe annulus center. Preferably, width L of the annular gap is at leastabout 0.25 inches (0.635 cm), but may be greater than about 0.5 inches(1.27 cm).

According to the present embodiment, at least one flow tube 50 extendsfrom outside stir chamber 10 to the inside of stir chamber 10, i.e.volume 35. Each flow tube should be constructed from a material capableof withstanding the high temperatures present in the chamber, typicallyin excess of 1400° C., without substantial degradation due to contactwith the oxide condensates. Each flow tube typically has an insidediameter of at least about 0.5 inches (1.27 cm), and is preferablycomprised of platinum, although other materials, such as a platinumalloy, may be used, provided the material exhibits a resistance tocorrosion or other forms of degradation (such as cracking) which mayresult from the harsh stir chamber environment.

Advantageously, end 52 of gas flow tube 50, which is within stir chamber10, is positioned such that it does not extend directly over glass freesurface 26 within the chamber. FIG. 1 depicts two gas flow tubes. Asshown by FIG. 1, gas flow tube ends 52 may terminate within an annularregion overtop the walls of the stir chamber rather than terminating ina position above free surface 26 of the glass melt. Thus, it is anadvantage of the embodiment that condensate which may accumulate at gasflow tube ends 52, and which may subsequently dislodge, is preventedfrom falling onto the glass free surface 26. Condensate falling onto thefree surface of the glass melt may result in contamination of the melt,and be manifest as inclusions or other defects within the glass articleproduced from the melt.

To overcome an upward flux of volatilized oxides through the annular gapbetween the stirrer shaft and the cover casing due to diffusion with adownward flux through the gap due to convection, the Peclet number,defined below, is preferably large in relation to the value 1:${Pe} = \frac{U \cdot L}{D}$In the above equation, Pe is the Peclet number, U is the downward gasvelocity in m/s, L is the width of the annulus between the cover casingand the outer surface of stir shaft 22 in meters, and D is the oxidediffusivity in m²/s. Condensation of volatile oxides along the stirrershaft, and in particular within and above annulus 38 surrounding thestir shaft, can be eliminated by causing a suitably high downward gasvelocity U. At the same time, it is also desirable to minimize the gasvelocity to reduce evaporation of oxides from the glass free surface asmuch as possible and to limit cooling of glass free surface 26 due tothe flow of gas through annulus 38. These competing requirements shouldbe reasonably balanced.

Another gas velocity limit exists due to instability from the opposingtemperature gradient. Since stir shaft 22 is cooler (typically by atleast about 800° C.) above chamber cover 32 than below the cover, aninstability in the gas flow through annulus 38 due to buoyancy may occurwhen there is an insufficient flow of gas through annulus 38. Convectioncells may develop within annulus 38 such that in some areas of theannulus the gas flow is upward and in others downward. Such convectioncells may aid the transport of volatile oxides into annulus 38, anddisrupt the flow of gas through the annulus. However, if gas velocity Uis sufficiently large in the downward direction, instability due tobuoyancy can be overcome

In one embodiment of the inventive method using the apparatus justdescribed, a vacuum is drawn on volume 35 above glass free surface 26through the at least one gas flow tube 50 of the apparatus by a suitablevacuum system (not shown), such as by a vacuum pump and associatedpiping. In other words, one end of gas flow tube 50 extends into volume35 within the stir chamber, while the opposite end is connected to asuitable vacuum system, the vacuum system therefore being in fluidcommunication with the inside of the stir chamber via gas flow tube 50.The vacuum pump may be, for example, a venturi pump driven by compressedair, although other vacuum pumps as are known in the art may be used.Preferably, the surfaces of the gas flow tube are at a temperature lowerthan the temperature of the molten glass surface, and preferably lowerthan the dew point of the volatile oxides within volume 35 so thatvolatized oxides removed from the stir chamber through gas flow tubes 50may condense within the flow tubes rather than on shaft 22 or on theinner surface of casing 36. The dew points of the volatile oxides in thevolume above the glass free surface depend on the glass composition andthe temperatures of the surfaces present within volume 35. Calculationsof the dew points of oxides along the shaft can be made based on thetemperature profile along the shaft, the diffusivity of a particularoxide, the gas flow velocity U through annular gap 38, and the dew pointvs. concentration curve for the oxide. Dew points for common volatileoxides may be as low as 559° C. for As₄O₆ to as high as 1455° C. forPtO₂.

A gas flow rate may be chosen which will maintain the oxide dew pointsbelow the shaft temperature at all points along the shaft. Since it ispreferable that volatilized oxides condense within the gas flow tuberather than along shaft 22, gas flow tube 50 may be designed such thatindividual flow tubes are replaceable and may further include filtrationto keep condensed oxides from fouling the vacuum system. Filtration maycomprise a stainless steel mesh or wool in a suitable can or containerthrough which the gas flows, such as are commercially available, forexample, through Nor-Cal Products, Inc.

The pressure differential between the reduced pressure within volume 35and the atmospheric pressure outside of the tank causes a flow ofatmospheric gas from outside the stir chamber through annular gap 38into volume 35. The velocity and volume of the gas flow is preferablysufficiently large that gas flow is everywhere downward in annulus 38(for the case where a vacuum is drawn on volume 35) in spite of adestabilizing temperature gradient which may be present. Additionally,the gas velocity should be sufficiently large so as to eliminatetransport of condensable materials upward, through the annular gap, bydiffusion. Preferably, the flow of atmospheric gas through annular gap38 is at least about 100 sccm, more preferably between about 400 sccmand about 900 sccm, although flow rates may be as high as 1600 sccm.

In another embodiment of the inventive apparatus, and as illustrated inFIG. 3, spacer plate 56 may be interposed between stir chamber wall 16and cover 32. Spacer plate 56 comprises at least one gas flow passage 58through the plate which serves the same function as the gas flow tubesin the previous embodiment. In the present embodiment, gas flow tubes 50may not be required. Spacer plate 56 preferably contains a plurality ofgas flow passages 58 disposed about the circumference of the plate. Theat least one gas flow passage may be connected individually to a vacuumsystem or a compressor system. However, manifold 60 may surround thespacer plate and connect the at least one gas flow passage to the vacuumsystem or the compressor system, depending upon whether a positivepressure or a negative pressure, with respect to atmospheric pressure,is desired in volume 35′ defined between cover 32, glass surface 26 andspacer 56. The use of a manifold may be desired when multiple gas flowpassages are utilized. As previously disclosed, a filter system isdesirable to remove condensate which may accumulate within the gas flowpassages or downstream piping. The apparatus of FIG. 3 is shown withoutgas flow tubes 50, however gas flow tubes 50 may also be used ifdesired. FIG. 4 shows spacer plate 56 and manifold 60 as a horizontalcross section, looking down on the spacer plate. Arrow 62 indicates gasflow into or out of the manifold, depending upon whether manifold 60 isconnected to a vacuum system for evacuating the stir chamber, or if themanifold is connected to a compressor system for pressurizing the stirchamber.

As in the case of the glass flow tubes of the previous embodiment, it ispreferable that the gas flow passage openings 64 which open into volume35 of the stir chamber are set back from the inside surface of stirchamber wall 16 so that any volatile oxides which may condense aroundthe openings does not fall into the molten glass within the stirchamber. A filter system as previously described may be suitablyinstalled, for example, in pipe 66 connecting manifold 60 with thevacuum system (no shown).

While various descriptions of the present invention are described above,it is understood that the various features described in connection withthe embodiments of the present invention can be used singly or incombination thereof. For example, gas flow passages could be formed inthe cover itself, or within an upper portion of the chamber wall.Therefore, this invention is not to be limited to the specificallypreferred embodiments depicted therein.

It will be apparent to those skilled in the art that various othermodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method of stirring a glass melt comprising: flowing molten glassthrough a stir chamber, the stir chamber having at least one wall and acover, the cover having a passage therethrough, the stir chamber furtherincluding a stirrer comprising a shaft which extends through the coverpassage, thereby forming an annular gap between the shaft and the cover;flowing a gas through the annular gap; and wherein the gas flows throughthe annular gap at a rate of at least about 100 sccm.
 2. The methodaccording to claim 1 wherein the gas is air.
 3. The method according toclaim 1 wherein the gas flow rate is at least about 400 sccm.
 4. Themethod according to claim 1 wherein the gas flow rate is at least about900 sccm.
 5. The method according to claim 1 further comprising heatingthe cover with a heater.
 6. The method according to claim 1 furthercomprising heating the shaft with a heater.
 7. The method according toclaim 1 further comprising heating the stir chamber by flowing a currentthrough an inside surface of the stir chamber.
 8. The method accordingto claim 1 wherein the step of flowing a gas comprises forming a vacuumwithin the stir chamber above a surface of the glass.
 9. The methodaccording to claim 1 wherein the step of flowing a gas comprisespressurizing the stir chamber.
 10. The method according to claim 8wherein the gas flow is everywhere downward through the annular gap. 11.The method according to claim 8 wherein the vacuum is formed byevacuating the chamber through at least one gas flow tube, the gas flowtube terminating at an end within the chamber which does not extend overa surface of a molten glass within the chamber.
 12. The method accordingto claim 9 wherein the chamber is pressurized through at least one gasflow tube, the gas flow tube terminating at an end within the chamberwhich does not extend over a surface of a molten glass within thechamber.
 13. The method according to claim 8 wherein the vacuum isformed by evacuating the chamber through at least one gas flow passage,the gas flow passage being located in a spacer plate between the chamberwall and the chamber cover.
 14. An apparatus for stirring a glass meltcomprising: a stir chamber configured to hold molten glass, the stirchamber including a cover defining a passage therethrough; a stirrerhaving a shaft extending through the passage into the stir chamber, thespace between the cover and the shaft defining an annular gap; at leastone gas flow tube for flowing a gas through the stir chamber, the gasflow tube having an end within the stir chamber, the end of the gas flowtube not extending overtop a surface of the molten glass.
 15. Theapparatus according to claim 14 wherein the gas flow tube is connectedwith a vacuum system.
 16. The apparatus according to claim 14 whereinthe gas flow tube is connected with a compressor.
 17. The apparatusaccording to claim 14 wherein a distance between a center of the annulargap and a center of the shaft is no more than about 0.15 inches.
 18. Amethod of stirring a glass melt using the apparatus of claim 14comprising: flowing molten glass through the stir chamber; drawing avacuum in the stir chamber through the gas flow tube; flowing a gasthrough the annular gap in a direction into the stir chamber; andwherein the gas flows through the annular gap at a rate of at leastabout 100 sccm.
 19. The method according to claim 18 wherein the gasflow rate is at least about 400 sccm.
 20. The method according to claim18 wherein the step of flowing a gas comprises forming a vacuum withinthe stir chamber.